The presently disclosed method and apparatus relates to circuits and systems for processing signals, and particularly with circuits and systems for constructing composite signals.
Composite signals are formed by assembling two or more signals into a combined signal spectrum. Such composite signals find utility in many applications. For example, systems used to distribute satellite television signals often employ means to construct composite signals. In such systems, various channels or bands of channels originating from several different satellites are assembled into a composite signal over which a user's set top box or other receiver can tune. Switch matrices are often used in such systems. A particular input signal (e.g., a Ku or Ka-band satellite signal) is supplied to an input of a switch matrix. The switch matrix provides the particular input signal to one or more of the switch matrix outputs. Two or more of such signals, each typically representing a different signal spectrum (i.e., containing different channels, or bands of channels) are combined (using, e.g., a diplexer or signal combiner network) and possibly frequency-translated to a second frequency (e.g., upper and lower L-band frequencies, 950 MHz-1450 MHz and 1650 MHz-2150 MHz). The combination of the two signals represent a composite signal that is supplied to a user for demodulation and/or baseband processing.
FIG. 1 illustrates a conventional satellite television distribution system in which a composite signal is generated and distributed. The system receives signals from two satellite signal sources and to output two composite signals. Each composite signal typically includes a portion of each of the two satellite signals. Each of the composite signals is supplied to a dual channel tuner (or two individual tuners). Each antenna receives two signals of different polarizations. Each polarization typically either has channel frequencies offset by half-channel width or has the same channel frequencies. In direct broadcast satellite (DBS) applications, the polarization is typically circular, having right-hand (R1 and R2) and left-hand (L1 and L2) polarized signals as labeled in FIG. 1. Signals can also be linearly polarized with horizontal and vertical polarizations.
The received signals are processed in a low noise block-converter 108 consisting of low noise amplifiers 107 (typically 2 or 3 amplifiers in a cascade), filters 109 (typically bandpass filters providing image rejection and reducing out of band power) and a frequency converter block 110. The converter block 110, performs frequency downconversion and contains local oscillators LO1 114 and LO2 112 (typically of the DRO (dielectric-resonator oscillator) types), mixers and post-mixer amplifiers. The two mixers driven by the local oscillator LO1 downconvert the signals to a lower (L) frequency band, while the mixers driven by the local oscillator LO2 downconvert to the signals to a higher (H) frequency band. The L and H bands are mutually exclusive (i.e., do not overlap) and have a frequency guard-band between them. The L and H band signals are then summed together in a separate signal combiner 116 in each arm, forming a composite signal having both frequency bands (i.e., “L+H”, which is often referred to as a “band-stacked signal” when the added signal components are bands of channels, or a “channel-stacked signal” when the added signal components are individual channels). The resulting sum is then coupled to a 2×4 switch matrix/converter block 120.
The switch matrix 130 routes each of the two input signals to selected one or more of the 4 outputs, either by first frequency converting the signals in the mixers 128 driven by LO3 132 or directly via the bypass switches around the mixers (the controls for the switch and mixer bypass not shown in the figure). The frequency of the LO3 is chosen such that the L-band converts into the H band, and vice versa, which is referred to as the “band-translation.” This is accomplished when the LO3 frequency is equal to the difference of the LO2 and LO1 frequencies.
The outputs of the matrix switch/converter block 120 are coupled through diplexers consisting of a high-pass filter 122, low-pass filter 124 and a combiner 126 (as shown in the upper arm, the lower arm being the same) providing two dual tuner outputs 118 and 134. The filters 122 and 124 remove the undesired portion of the spectrum, i.e. the unwanted bands in each output. Each of the two outputs 118 and 134 feeds via a separate coaxial cable a dual tuner, for a total capability of four tuners. By controlling the matrix switch routing and the mixer conversion/bypass modes, a frequency translation is accomplished and each of the four tuners can independently tune to any of the channels from either polarization of either satellite.
While operational, the conventional system suffers from some disadvantages, one of which is a relatively low source-to-source isolation. In particular, the low noise converter block 108 and the switch matrix converter block 120 each may exhibit low isolation between their respective signal paths. This may lead to cross-coupling of the signals and contamination of the composite signal with unwanted signal content. This cross-coupling effect becomes especially acute when the sources operate at high frequencies and over the same band. Such conditions which exist in the aforementioned satellite TV distribution system, in which both satellite sources operate over the same Ku or Ka-band.
Another disadvantage of the conventional system is that multiple frequency translations are needed to provide the desired composite output signal. In particular, the low noise block converter 108 provides a first frequency translation, e.g., to downconvert the received satellite signal from Ku-band to L-band, and the switch matrix/converter 120 provides a second frequency translation, e.g., to translate the downconverted signal from a lower band to an upper band, or vice versa. Multiple frequency conversions increase the system's complexity, cost, and power consumption, as well as degrade signal quality.